Surface morphology of non-polar gallium nitride containing substrates

ABSTRACT

Optical devices such as LEDs and lasers are discloses. The devices include a non-polar gallium nitride substrate member having an off-axis non-polar oriented crystalline surface plane. The off-axis non-polar oriented crystalline surface plane can be up to about −0.6 degrees in a c-plane direction and up to about −20 degrees in a c-plane direction in certain embodiments. In certain embodiments, a gallium nitride containing epitaxial layer is formed overlying the off-axis non-polar oriented crystalline surface plane. In certain embodiments, devices include a surface region overlying the gallium nitride epitaxial layer that is substantially free of hillocks.

This application is a continuation of U.S. application Ser. No.13/621,485, filed on Sep. 17, 2012, now allowed, which is a continuationin part of U.S. application Ser. No. 13/548,635 filed on Jul. 13, 2012,issued as U.S. Pat. No. 8,575,728, on Nov. 5, 2013, which is acontinuation of U.S. application Ser. No. 12/497,289 filed on Jul. 2,2009, which issued as U.S. Pat. No. 8,247,887 on Aug. 21, 2012, andwhich claims the benefit under 35 U.S.C. §119(e) of U.S. ProvisionalApplication No. 61/182,107 filed on May 29, 2009, each of which isincorporated by reference in its entirety.

BACKGROUND

The present invention is directed to optical devices and relatedmethods. More particularly, the present invention provides a method anddevice for fabricating crystalline films for emitting electromagneticradiation using non-polar gallium containing substrates such as GaN, MN,InN, InGaN, AlGaN, and AlInGaN, and others. Merely by way of example,the invention can be applied to optical devices, lasers, light emittingdiodes, solar cells, photoelectrochemical water splitting and hydrogengeneration, photodetectors, integrated circuits, and transistors, amongother devices.

In the late 1800's, Thomas Edison invented the conventional light bulb.The conventional light bulb, commonly called the “Edison bulb,” has beenused for over one hundred years for a variety of applications includinglighting and displays. The conventional light bulb uses a tungstenfilament enclosed in a glass bulb sealed in a base, which is screwedinto a socket. The socket is coupled to an AC power or DC power source.The conventional light bulb can be found commonly in houses, buildings,and outdoor lightings, and other areas requiring light or displays.Unfortunately, drawbacks exist with the conventional Edison light bulb.

First, the conventional light bulb dissipates much thermal energy. Morethan 90% of the energy used for the conventional light bulb dissipatesas thermal energy.

Secondly, reliability is an issue since the conventional light bulbroutinely fails often due to thermal expansion and contraction of thefilament element.

Thirdly, light bulbs emit light over a broad spectrum, much of whichdoes not result in bright illumination due to the spectral sensitivityof the human eye.

Lastly, light bulbs emit in all directions and are not ideal forapplications requiring strong directionality or focus such as projectiondisplays, optical data storage, or specialized directed lighting.

In 1960, Theodore H. Maiman demonstrated the first laser at HughesResearch Laboratories in Malibu. This laser utilized a solid-stateflashlamp-pumped synthetic ruby crystal to produce red laser light at694 nm. By 1964, blue and green laser output was demonstrated by WilliamBridges at Hughes Aircraft utilizing a gas laser design called an Argonion laser. The Ar-ion laser utilized a noble gas as the active mediumand produce laser light output in the UV, blue, and green wavelengthsincluding 351 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm,496.5 nm, 501.7 nm, 514.5 nm, and 528.7 nm. The Ar-ion laser had thebenefit of producing highly directional and focusable light with anarrow spectral output, but the wall plug efficiency was <0.1%, and thesize, weight, and cost of the lasers were undesirable as well.

As laser technology evolved, more efficient lamp pumped solid statelaser designs were developed for the red and infrared wavelengths, butthese technologies remained a challenge for blue and green lasers. As aresult, lamp pumped solid state lasers were developed in the infrared,and the output wavelength was converted to the visible using specialtycrystals with nonlinear optical properties. A green lamp pumped solidstate laser had 3 stages: electricity powers lamp, lamp excites gaincrystal which lases at 1064 nm, 1064 nm goes into frequency conversioncrystal which converts to visible 532 nm. The resulting green and bluelasers were called “lamped pumped solid state lasers with secondharmonic generation” (LPSS with SHG) had wall plug efficiency of ˜1%,and were more efficient than Ar-ion gas lasers, but were still tooinefficient, large, expensive, fragile for broad deployment outside ofspecialty scientific and medical applications. Additionally, the gaincrystal used in the solid state lasers typically had energy storageproperties which made the lasers difficult to modulate at high speedswhich limited its broader deployment.

To improve the efficiency of these visible lasers, high power diode (orsemiconductor) lasers were utilized. These “diode pumped solid statelasers with SHG” (DPSS with SHG) had 3 stages: electricity powers 808 nmdiode laser, 808 nm excites gain crystal which lases at 1064 nm, 1064 nmgoes into frequency conversion crystal which converts to visible 532 nm.The DPSS laser technology extended the life and improved the wall plugefficiency of the LPSS lasers to 5-10%, and further commercializationensue into more high end specialty industrial, medical, and scientificapplications. However, the change to diode pumping increased the systemcost and required precise temperature controls, leaving the laser withsubstantial size, power consumption while not addressing the energystorage properties which made the lasers difficult to modulate at highspeeds.

As high power laser diodes evolved and new specialty SHG crystals weredeveloped, it became possible to directly convert the output of theinfrared diode laser to produce blue and green laser light output. These“directly doubled diode lasers” or SHG diode lasers had 2 stages:electricity powers 1064 nm semiconductor laser, 1064 nm goes intofrequency conversion crystal which converts to visible 532 nm greenlight. These lasers designs are meant to improve the efficiency, costand size compared to DPSS-SHG lasers, but the specialty diodes andcrystals required make this challenging today. Additionally, while thediode-SHG lasers have the benefit of being directly modulate-able, theysuffer from severe sensitivity to temperature which limits theirapplication. These and other limitations may be described throughout thepresent specification and more particularly below.

From the above, it is seen that techniques for improving optical devicesis highly desired.

BRIEF SUMMARY

According to the present invention, techniques related generally tooptical devices are provided. More particularly, the present inventionprovides a method and device for fabricating crystalline films foremitting electromagnetic radiation using non-polar gallium containingsubstrates such as GaN, MN, InN, InGaN, AlGaN, and AlInGaN, and others.Merely by way of example, the invention can be applied to opticaldevices, lasers, light emitting diodes, solar cells,photoelectrochemical water splitting and hydrogen generation,photodetectors, integrated circuits, and transistors, among otherdevices. In a preferred embodiment, the optical device is a laser thathas been configured for blue and green emissions, as well as others. Instill a preferred embodiment, the optical device is an LED that has beenconfigured for blue emission, as well as others.

In a specific embodiment, the present invention provides a method andresulting nonpolar m-plane (10-10) oriented gallium nitride structurehaving smooth surface morphology, which is often substantially free fromhillocks and the like. In one or more embodiments, the method includesusing a miscut or offcut surface or no miscut or offcut or otheroff-axis orientation of a non-polar m-plane surface orientation as agrowth surface region. In a preferred embodiment, the epitaxial layer isconfigured using at least an atmospheric pressure (e.g., 650-850 Torr)epitaxial formation process, but may also be configured for otherprocesses. In a specific embodiment, the method includes use of a N₂carrier and subflow gas, which is substantially all N₂, as a medium forprecursor gases, which form the crystalline gallium nitride epitaxialmaterial. The growth using the substantially predominant N₂ gas leads toformation of crystalline gallium nitride epitaxial materialsubstantially free of hillocks and the like. As used herein, the term“nonpolar (10-10) oriented gallium nitride structure” refers to thefamily of nonpolar m-plane (10-10) oriented gallium nitride structuresand the like.

In an alternative preferred embodiment, the present invention includesuse of a gallium nitride substrate configured in a non-polar (10-10)surface orientation that has a miscut toward the c-plane (0001) rangingfrom about −0.6 degrees to about −2.0 degrees and any miscut toward thea-plane (11-20) although there can be other orientations and degrees ofmiscut or offcut or off-axis orientation. In one or more embodiments,the method uses an H₂ carrier gas and combination of H₂ and N₂ subflowgases in further combination with precursor gases for growth ofcrystalline gallium nitride epitaxial material. In still a preferredembodiment, the miscut can be about −0.8 degrees to about −1.1 degreestoward the c-plane (0001) and between −0.3 degrees and 0.3 degreestoward the a-plane (11-20) to cause formation of an overlying galliumnitride epitaxial layer with smooth morphology.

Still further, the present invention provides an optical device that hasepitaxial film that is substantially free from morphological features onthe surface such as hillocks and the like. In a specific embodiment thedevice has a non-polar (10-10) gallium nitride substrate member having aslightly off-axis non-polar oriented crystalline surface plane. In oneor more embodiments, the slightly off-axis (or on-axis) non-polaroriented crystalline surface plane ranges from about 0 degrees to apredetermined degree toward either or both the c-plane and/or a-plane.In a specific embodiment, the device has a gallium nitride containingepitaxial layer formed overlying the slightly off-axis non-polaroriented crystalline surface plane. A surface region is overlying thegallium nitride epitaxial layer. In a preferred embodiment, the surfaceregion being substantially free from hillocks having an average spatialdimension of, for example, 10-100 microns and greater, but can be otherdimensions. In a preferred embodiment, the epitaxial layer is configuredusing at least an atmospheric pressure (e.g., 650-850 Torr) epitaxialformation process. In a specific embodiment, the epitaxial layercomprises one or more layers which form at least a quantum well of atleast 1.5 nanometers and greater or at least 3.0 nm or at least 5.0 nmnanometers and greater. The quantum well, which is thicker, leads toimproved laser devices.

Still further, the present invention provides a method of fabricating anoptical device. The method includes providing a non-polar (10-10)gallium nitride substrate member having a slightly off-axis non-polaroriented crystalline surface plane. In a specific embodiment, theslightly off-axis non-polar oriented crystalline surface plane isgreater in magnitude than about negative 0.6 degrees toward the c-plane(0001). The method includes forming a gallium nitride containingepitaxial layer having a smooth surface region substantially free ofhillocks overlying the slightly off-axis non-polar oriented crystallinesurface plane.

In yet other embodiments, the present invention provides a method offabricating an alternative optical device. The method includes providinga non-polar (10-10) gallium nitride substrate member having a slightlyoff-axis non-polar oriented crystalline surface plane in a specificembodiment. The slightly off-axis non-polar oriented crystalline surfaceplane ranges from about 0 degrees to a predetermined degree towardeither or both the c-plane or a-plane. In a specific embodiment, thepresent method includes forming a gallium nitride containing epitaxiallayer, using at least an atmospheric pressure (e.g., 700-800 Torr)epitaxial process to form at least a quantum well having a thickness ofat least 1.5 or 3.5 nanometers and greater. Preferably, the galliumnitride epitaxial layer has a surface region substantially smooth andfree from hillocks.

In a specific embodiment, the present invention provides a method andresulting in an off-cut from an m-plane (10-10) oriented gallium nitridestructure having smooth surface morphology, which is often substantiallyfree from hillocks and the like. In an example, the off-cut can be up toabout +/−21 degrees toward a c-plane and/or +/−10 degrees toward ana-plane, such that the orientations may be considered an exposedsemi-polar plane. Examples of such semipolar planes would (30-31),(30-3-1), (20-21), (20-2-1), (30-32), and (30-3-2), among others. In apreferred embodiment, the epitaxial layer is configured using at leastan atmospheric pressure (e.g., 650-850 Torr) epitaxial formationprocess, but may also be configured for other processes. In a specificembodiment, the method includes use of a N₂ carrier and subflow gas,which is substantially all N₂, as a medium for precursor gases, whichform the crystalline gallium nitride epitaxial material. The growthusing the substantially predominant N₂ gas leads to formation ofcrystalline gallium nitride epitaxial material substantially free ofhillocks and the like.

In yet other embodiments, the present invention provides a method offabricating an alternative optical device. The method includes providinga non-polar (10-10) m-plane gallium and nitrogen containing substratemember having an off-cut to provide an off-axis oriented crystallinesurface plane in a specific embodiment. In an example, the off-cut oroff-set can be up to about +/−20 degrees toward a c-plane and/or +/−10degrees toward an a-plane, such that the orientations may be consideredan exposed semi-polar plane. Examples of such semipolar planes would(30-31), (30-3-1), (20-21), (20-2-1), (30-32), and (30-3-2), amongothers. In a specific embodiment, the present method includes forming agallium nitride containing epitaxial layer, using at least anatmospheric pressure (e.g., 650-850 Torr) epitaxial process to form atleast a quantum well having a thickness of at least 1.5, 3.5, or 5.0nanometers and greater. Preferably, the gallium nitride epitaxial layerhas a surface region substantially smooth and free from hillocks.

In yet other embodiments, the present invention provides a method offabricating an alternative optical device. The method includes providinga non-polar (10-10) m-plane gallium and nitrogen containing substratemember having an off-cut to provide an off-axis oriented crystallinesurface plane in a specific embodiment. In an example, the off-cut oroff-set can be up to about +/−20 degrees toward a c-plane and/or +/−10degrees toward an a-plane, such that the orientations may be consideredan exposed semi-polar plane. Examples of such semipolar planes would(30-31), (30-3-1), (20-21), (20-2-1), (30-32), and (30-3-2), amongothers. In a specific embodiment, the present method includes forming agallium and nitrogen containing epitaxial layer using an epitaxialprocess with greater than atmospheric pressure of 800-1600 Torr to format least a quantum well having a thickness of at least 1.5, 3.5, or 5.0nanometers and greater. Preferably, the gallium nitride epitaxial layerhas a surface region substantially smooth and free from hillocks.

As used herein, the term “miscut” should be interpreted according toordinary meaning understood by one of ordinary skill in the art and doesnot imply a specific process to achieve the orientation. The term miscutis not intended to imply any undesirable cut relative to, for example,any of the crystal planes, e.g., c-plane, a-plane. The term miscut isintended to describe a surface orientation slightly tilted with respectto the primary surface crystal plane such as the nonpolar (10-10) GaNplane, however, does not need to physically originate from the primarysurface crystal plane, which is merely a reference point. Additionally,the term “offcut” or “off-axis” is intended to have a similar meaning asmiscut that does not imply any process to achieve the orientation,although there could be other variations, modifications, andalternatives. In yet other embodiments, the crystalline surface plane isnot miscut and/or offcut and/or off-axis but can be configured using amechanical and/or chemical and/or physical process to expose any one ofthe crystalline surfaces described explicitly and/or implicitly herein.In specific embodiments, the terms miscut and/or offcut and/or off-axisare characterized by at least one or more directions and correspondingmagnitudes, although there can be other variations, modifications, andalternatives.

Benefits are achieved over pre-existing techniques using the presentinvention. In particular, the present invention enables a cost-effectivetechnique for growth of large area crystals of non-polar materials,including GaN, AlN, InN, InGaN, and AlInGaN and others. In a specificembodiment, the present method and resulting structure are relativelysimple and cost effective to manufacture for commercial applications. Aspecific embodiment also takes advantage of a combination of techniques,which solve a long standing need. In a preferred embodiment, the (10-10)non-polar substrate and overlying epitaxial crystalline gallium nitridecontaining film are smooth and substantially free from hillocks and thelike, which improve device performance. As used herein, the term“smooth” generally means substantially free from hillocks or othersurface imperfections, which lead to degradation in device performance,including reliability, intensity, efficiency, and other parameters thatgenerally define performance. Of course, the term smooth would alsoinclude other interpretations known by one of ordinary skill in the art,as well as variations, modifications, and alternatives. Depending uponthe embodiment, one or more of these benefits may be achieved. These andother benefits may be described throughout the present specification andmore particularly below.

The present invention achieves these benefits and others in the contextof known process technology. However, a further understanding of thenature and advantages of the present invention may be realized byreference to the latter portions of the specification and attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an optical micrograph image representative of a conventionalsurface region including hillock structures on a non-polar GaNsubstrate.

FIG. 1B is a schematic illustration of a top down view of a hillockstructure.

FIG. 1C is a schematic illustration of a cross-sectional view of ahillock structure.

FIG. 2 is a simplified flow diagram of a method for fabricating animproved GaN film according to an embodiment of the present invention.

FIG. 3 is a simplified diagram illustrating various miscuts/offcuts in+/−c/a planes according to one or more embodiments of the presentinvention.

FIG. 4A and FIG. 4B present optical micrograph images of the resultingsurface morphology in epitaxial films grown with the use of H₂ carriergas.

FIG. 5 shows positive and negative miscuts toward the c-plane (0001)were explored using growth techniques with H₂ as the carrier gas.

FIG. 6A, FIG. 6B, and FIG. 6C show the choice of MOCVD carrier gas.

FIG. 7A and FIG. 7B show an effect of large negative miscut of about 5degrees toward (0001) according to an example of the present invention.

FIG. 8A and FIG. 8B show an effect of large negative miscut of ˜7degrees toward (0001) according to an example of the present invention.

FIG. 9A and FIG. 9B show an effect of large negative miscut of about 10degrees toward (0001) according to an example of the present invention.

FIG. 10A and FIG. 10B show an effect of large negative miscut of about15 degrees toward (0001) according to an example of the presentinvention.

FIG. 11A and FIG. 11B show an effect of large positive miscut of about10 degrees toward (0001) according to an example of the presentinvention.

FIG. 12A and FIG. 12B show an effect of large positive miscut of about15 degrees toward (0001) according to an example of the presentinvention.

FIG. 13 shows an effect of large positive miscut of about 20 degreestoward (0001) according to an example of the present invention.

FIG. 14A and FIG. 14B shows AFM surface morphology of growth for H₂ andN₂ carrier gas on (20-21) according to an example of the presentinvention.

FIG. 15A and FIG. 15B shows AFM surface morphology for growth with N₂carrier gas on (20-21) according to an example of the present invention.

FIG. 16A and FIG. 16B shows AFM surface morphology for multilayer growthusing a first N₂ carrier gas and a second H2 carrier gas on (20-21)according to an example of the present invention.

DETAILED DESCRIPTION

According to the present invention, techniques related generally tooptical devices are provided. More particularly, the present inventionprovides a method and device for fabricating crystalline films foremitting electromagnetic radiation using non-polar (10-10) galliumcontaining substrates such as GaN, MN, InN, InGaN, AlGaN, and AlInGaN,and others. Merely by way of example, the invention can be applied tooptical devices, lasers, light emitting diodes, solar cells,photoelectrochemical water splitting and hydrogen generation,photodetectors, integrated circuits, and transistors, among otherdevices.

In one or more embodiments, the present invention is directed togenerate high efficiency GaN-based light emitting devices operating atwavelengths beyond 400 nm for blue, green, yellow and red emissionaccording to embodiments of the present invention. The proposed devicewill be used as an optical source for various commercial, industrial, orscientific applications. These structures are expected to find utilityin existing applications where blue-violet, blue, green, yellow and redlaser/LED emission is required. Existing applications include displaysystems based on blue and/or green laser diodes and violet laser diodesfor HD-DVD and Sony Blu-Ray™ players. One particularly promisingapplication for these devices is specialty lighting where blue laserdiodes will pump phosphors to emit white light. Laser based televisionis also expected to emerge in coming year. Other potential applicationis for optical communication through polymer based fibers or underwatercommunication.

In a specific embodiment, the present invention provides a GaN-basedsemiconductor laser/LED growth/fabrication method to achieve increasedwavelength operation into the blue, green, yellow and red regime onnonpolar GaN substrates where superior laser/LED performance can beexpected according to a specific embodiment. The device relies on smoothsurface region films of epitaxial crystalline GaN containing materialsfor improved device performance. The smooth surface region and thereforehigher quality crystalline material can be derived from epitaxial growthtechniques according to one or more embodiments.

Epitaxial growth on the nonpolar (10-10) plane of bulk GaN has beenemerging and possesses various limitations. Understanding growthparameter space for optimal epitaxial layer deposition is oftenimportant for the realization of high performance electronic onoptoelectronic devices fabricated from the epitaxial layers. At leastone key aspect of the film quality is the morphology. Morphologymanifests itself in large scale features that are on the order of tensto hundreds of microns all the way down to the atomic scale on the orderof Angstroms. Achieving smooth epitaxial layers on both the large scaleand small scale often translate into high performance devices.

In a specific embodiment, the present invention provides a method offabricating an optical device. The method includes providing a non-polar(10-10) gallium nitride substrate member having an off-axis non-polaroriented crystalline surface plane, which is greater in magnitude thanabout negative 0.6 degrees and less than 20 degrees toward the c-plane(0001). The method includes forming a gallium nitride containingepitaxial layer having a surface region substantially free of hillocksoverlying the slightly off-axis non-polar oriented crystalline surfaceplane and maintaining the gallium nitride containing epitaxial layer inan atmospheric environment during the formation of the gallium andnitrogen containing epitaxial layer.

In an alternative embodiment, the present invention provides an opticaldevice. The optical device includes a gallium containing substratemember having an off-axis m-plane oriented crystalline surface plane.The off-axis m-plane oriented crystalline surface plane ranging fromabout 0 degrees to about +/−20 degrees toward a c-plane and a galliumnitride containing epitaxial layer formed overlying the off-axisoriented crystalline surface plane configured using at least asubstantially atmospheric pressure epitaxial formation process to format least a region of a quantum well of at least 1.5 nanometers andgreater. The device includes a surface region overlying the galliumnitride epitaxial layer, the surface region being substantially freefrom hillocks.

In an alternative example, the present invention provides a method offabricating a laser device configured to emit electromagnetic radiationranging from 420 nm to 485 nm or 500 nm to 550 nm. The method includesproviding a non-polar (10-10) gallium nitride substrate member having anoff-axis non-polar oriented crystalline surface plane. The off-axisnon-polar oriented crystalline surface plane is between negative 13 andnegative 17 degrees from an m-plane toward a c-plane and the off-axisnon-polar oriented crystalline surface plane being between 2 and 8degrees from m-plane toward an a-plane. The method includes forming agallium nitride containing epitaxial layer having a surface regionsubstantially free of hillocks overlying the slightly off-axis non-polaroriented crystalline surface plane under an atmospheric environmentranging from 650 Torr to 850 Torr during the formation of the galliumand nitrogen containing epitaxial layer. The method also includesforming a ridge structure configured overlying the gallium nitridecontaining epitaxial layer such that the ridge structure is aligned in aprojection of a c-direction.

In a specific example, the present invention provides a method offabricating a laser device configured to emit electromagnetic radiationranging from 420 nm to 485 nm or 500 nm to 550 nm. The method includesproviding a non-polar (10-10) gallium and nitrogen containing substratemember having an off-axis non-polar oriented crystalline surface plane,which is greater in magnitude than about 0.6 degrees and less inmagnitude than about 20 degrees from an m-plane toward a c-plane. Themethod includes forming a gallium and nitrogen containing epitaxiallayer formed overlying the off-axis oriented crystalline surface planeconfigured using at least a substantially atmospheric pressure epitaxialformation process to form at least a region of a quantum well having athickness of at least 1.5 nanometers and greater. The method includesforming a surface region overlying the gallium and nitrogen containingepitaxial layer. The method includes forming a gallium and nitrogencontaining epitaxial layer having a surface region substantially free ofhillocks overlying the off-axis non-polar oriented crystalline surfaceplane and forming at least a quantum well region under a superatmospheric pressure environment ranging from about 800 to about 1600Torr. The quantum well region has a thickness of at least 1.5 nanometersand greater. The method includes forming a ridge structure configuredoverlying the gallium nitride containing epitaxial layer such that theridge structure is aligned in a projection of a c-direction.

FIG. 1A is an optical micrograph image which represents a conventionalsurface region of a non-polar (10-10) oriented gallium nitride epitaxiallayer, including hillock structures. The surface shown is representativeof epitaxial deposition at atmospheric pressure conditions (e.g.,700-800 Torr) on a non-polar (10-10) GaN substrate. As shown, non-polarGaN can exhibit very distinct large-scale features referred to herein ashillocks. FIG. 1B and FIG. 1C are schematic illustrations of a top-downand cross-sectional view of such a hillock feature. As shown, thesehillocks are pyramidal in shape and typically elongated in the in thepositive and negative a-directions and can demonstrate significantlysteep sidewalls in the positive and negative c-directions. Lateraldimensions of such hillocks can range from 50-100 microns or greater.The hillocks can have a height scale on the orders of hundreds ofnanometers, therefore they can be disruptive/detrimental tooptoelectronic devices such as laser diodes since the cladding layerswill have varying thickness along the cavity and the gain layers betweenthe cladding layers can have sharp interfaces. As shown, the large-scalemorphological features are predominantly “pyramidal hillocks” or likestructures. These characteristics can lead to increased loss in opticaldevices such as lasers, reduced gain, and perhaps reduced yield andreliability.

A method according to one or more embodiments for forming a smoothepitaxial film using an offcut or miscut or off-axis substrate isbriefly outlined below.

-   -   1. Provide GaN substrate or boule;    -   2. Perform off-axis miscut of GaN substrate on nonpolar        crystalline planes to expose desired surface region or process        substrate or boule (e.g., mechanical process) to expose off-axis        oriented surface region from the nonpolar (10-10) m-plane;    -   3. Transfer GaN substrate into MOCVD process chamber;    -   4. Provide a carrier gas selected from nitrogen gas, hydrogen        gas, or a mixture of them;    -   5. Provide a nitrogen bearing species such as ammonia or the        like;    -   4. Raise MOCVD process chamber to growth temperature, e.g., 700        to 1200 Degrees Celsius and configured at atmospheric pressure,        above atmospheric pressure, or reduced pressure;    -   5. Maintain the growth temperature within a predetermined range;    -   6. Combine the carrier gas and nitrogen bearing species such as        ammonia with group III precursors such as the indium precursor        species tri-methyl-indium and/or tri-ethyl-indium, the gallium        precursor species tri-methyl-gallium and/or tri-ethyl-gallium,        and/or the aluminum precursor tri-methyl-aluminum into the        chamber;    -   7. Form an epitaxial film containing one or more of the        following layers GaN, InGaN, AlGaN, InAlGaN;    -   8. Cause formation of a surface region of the epitaxial gallium        nitride film substantially free from hillocks and other surface        roughness structures and/or features;    -   9. Repeat steps (7) and (8) for other epitaxial films to form        one or more device structures; and    -   10. Perform other steps, desired.

The above sequence of steps provides a method according to an embodimentof the present invention. In a specific embodiment, the presentinvention provides a method and resulting crystalline epitaxial materialwith a surface region that is substantially smooth and free fromhillocks and the like for improved device performance. Although theabove has been described in terms of an off-axis surface configuration,there can be other embodiments having an on-axis configuration using oneor more selected process recipes, which have been described in moredetail throughout the present specification and more particularly below.Other alternatives can also be provided where steps are added, one ormore steps are removed, or one or more steps are provided in a differentsequence without departing from the scope of the claims herein.

As merely an example, the present method can use the following sequenceof steps in forming one or more of the epitaxial growth regions using anMOCVD tool operable at atmospheric pressure or low pressure in someembodiments.

-   -   1. Start;    -   2. Provide a crystalline substrate member comprising a backside        region and a surface region, which has been offcut or miscut or        off-axis;    -   3. Load substrate member into an MOCVD chamber;    -   4. Place substrate member on susceptor, which is provided in the        chamber, to expose the offcut or miscut or off axis surface        region of the substrate member;    -   5. Subject the surface region to a first flow (e.g., derived        from one or more precursor gases including at least an ammonia        containing species, a Group III species, and a first carrier        gas) in a first direction substantially parallel to the surface        region;    -   6. Form a first boundary layer within a vicinity of the surface        region;    -   7. Provide a second flow (e.g., derived from at least a second        carrier gas) in a second direction configured to cause change in        the first boundary layer to a second boundary layer;    -   8. Increase a growth rate of crystalline material formed        overlying the surface region of the crystalline substrate        member;    -   9. Continue crystalline material growth to be substantially free        from hillocks and/or other imperfections;    -   10. Cease flow of precursor gases to stop crystalline growth;    -   11. Perform other steps and repetition of the above, as desired;    -   12. Stop.

The above sequence of steps provides methods according to an embodimentof the present invention. As shown, the method uses a combination ofsteps including a way of forming a film of crystalline material usingMOCVD. In preferred embodiments, the present invention includes amultiflow technique provided at atmospheric pressure (e.g., 700-800Torr) for formation of high quality gallium nitride containingcrystalline films that are substantially free from hillocks and otherimperfections that lead to crystal degradation. Many other methods,devices, and systems are also included. Of course, other alternativescan also be provided where steps are added, one or more steps areremoved, or one or more steps are provided in a different sequencewithout departing from the scope of the claims herein. Additionally, thevarious methods can be implemented using a computer code or codes insoftware, firmware, hardware, or any combination of these. In otherembodiments, the present MOCVD tool can be modified, updated, varied, orcombined with other hardware, processing, and software. Further detailsof the present method can be found throughout the present specificationand more particularly below in reference to the figures.

FIG. 2 is a simplified flow diagram of a method for fabricating animproved GaN film according to an embodiment of the present invention.In a specific embodiment, the present method uses a technique usingMOCVD as described in, for example, U.S. application Ser. No. 12/573,820filed on Oct. 5, 2009, which is incorporated by reference herein.

As shown, the present method begins with start, step 201. In a specificembodiment, the present method uses a MOCVD reactor configured to carryout the present method. Details of the reactor are provided moreparticularly in U.S. application Ser. No. 12/573,820 filed on Oct. 5,2009, which is incorporated by reference herein.

In a specific embodiment, the present invention provides (step 203) acrystalline substrate member comprising a backside region and a surfaceregion. In a specific embodiment, the crystalline substrate member caninclude, among others, a gallium nitride wafer, or the like. Morepreferably, the substrate is an offcut of the nonpolar (10-10) m-planeGaN substrate, but can be others. In an example, the off-cut or off-setcan be up to about +/−21 degrees toward a c-plane and/or +/−10 degreestoward an a-plane, such that the orientations may be considered anexposed semi-polar plane. Examples of such semipolar planes would(30-31), (30-3-1), (20-21), (20-2-1), (30-32), and (30-3-2), amongothers.

In a specific embodiment, the present method uses a miscut or offcutcrystalline substrate member or boule of GaN, but can be other materialsand does not imply use of a process of achieving the miscut or offcut.As used herein, the term “miscut” should be interpreted according toordinary meaning as understood by one of ordinary skill in the art. Theterm miscut is not intended to imply any undesirable cut relative to,for example, any of the crystal planes, e.g., c-plane, a-plane. The termmiscut is intended to describe a surface orientation slightly tiltedwith respect to the primary surface crystal plane such as the nonpolar(10-10) GaN plane. As used herein, the term “slightly” can include anoffcut toward the c-plane at values of up to about +/−21 degrees towarda c-plane and/or +/−10 degrees toward an a-plane, such that theorientations may be considered semipolar. Examples of such semipolarplanes include (30-31), (30-3-1), (20-21), (20-2-1), (30-32), and(30-3-2), among others. Additionally, the term “offcut” is intended tohave a similar meaning as miscut, although there could be othervariations, modifications, and alternatives. In yet other embodiments,the crystalline surface plane is not miscut and/or offcut but can beconfigured using a mechanical and/or chemical and/or physical process toexpose any one of the crystalline surfaces described explicitly and/orimplicitly herein. In specific embodiments, the term miscut and/oroffcut and/or off axis is characterized by at least one or moredirections and corresponding magnitudes, although there can be othervariations, modifications, and alternatives.

As shown, the method includes placing or loading (step 205) thesubstrate member into an MOCVD chamber. In a specific embodiment, themethod supplies one or more carrier gases, step 207, and one or morenitrogen-bearing precursor gases, step 209, which are described in moredetail below. In one or more embodiments, the crystalline substratemember is provided on a susceptor from the backside to expose thesurface region of the substrate member. The susceptor is preferablyheated using resistive elements or other suitable techniques. In aspecific embodiment, the susceptor is heated (step 211) to a growthtemperature ranging from about 700 to about 1200 Degrees Celsius, butcan be others.

In a specific embodiment, the present method includes subjecting thesurface region of the crystalline substrate to a first flow in a firstdirection substantially parallel to the surface region. In a specificembodiment, the method forms a first boundary layer within a vicinity ofthe surface region. In a specific embodiment, the first boundary layeris believed to have a thickness ranging from about 1 millimeters toabout 1 centimeters, but can be others. Further details of the presentmethod can be found below.

Depending upon the embodiment, the first flow is preferably derived fromone or more precursor gases including at least an ammonia containingspecies, a Group III species (step 213), and a first carrier gas, andpossibly other entities. Ammonia is a Group V precursor according to aspecific embodiment. Other Group V precursors include N₂. In a specificembodiment, the first carrier gas can include hydrogen gas, nitrogengas, argon gas, or other inert species, including combinations. In aspecific embodiment, the Group III precursors include TMGa, TEGa, TMIn,TMAl, dopants (e.g., Cp₂Mg, disilane, silane, diethelyl zinc, iron,manganese, or cobalt containing precursors), and other species. Asmerely an example, a preferred combination of miscut/offcut/substratesurface configurations, precursors, and carrier gases are providedbelow.

-   -   Off-cut non-polar (10-10) GaN substrate surface configured less        than +/1 20 degrees in magnitude toward c-plane (0001);    -   Carrier Gas: Any mixture of N₂ and H₂, but preferably all H₂;    -   Group V Precursor: NH₃; Group III Precursor: TMGa and/or TEGa        and/or TMIn and/or TEIn and/or TMAl; and    -   Optional Dopant Precursor: Disilane, silane, Cp₂Mg;    -   or    -   Off-cut non-polar (10-10) GaN substrate surface configured less        than +/1 20 degrees in magnitude toward c-plane (0001);    -   Carrier Gas: Any mixture of N₂ and H₂, but preferably all N₂;    -   Group V Precursor: NH₃; Group III Precursor: TMGa and/or TEGa        and/or TMIn and/or TEIn and/or TMAl; and    -   Optional Dopant Precursor: Disilane, silane, Cp₂Mg;

In a specific embodiment, the present method also includes a step ofproviding a second flow (e.g., derived from at least a second carriergas) in a second direction configured to cause change in the firstboundary layer to a second boundary layer. In a specific embodiment, thesecond direction is normal to the first direction, but can be slightlyvaried according to other embodiments. Additionally, the second boundarylayer facilitates improved crystalline growth as compared to formationusing the first boundary layer embodiment. In a specific embodiment, thesecond flow increases a growth rate of crystalline material formedoverlying the surface region of the crystalline substrate member.

Depending upon the embodiment, the method also continues (step 215) withepitaxial crystalline material growth, which is substantially smooth andfree of hillocks or other imperfections. In a specific embodiment, themethod also can cease flow of precursor gases to stop growth and/orperform other steps. In a specific embodiment, the method stops at step217. In a preferred embodiment, the present method causes formation of agallium nitride containing crystalline material that has a surfaceregion that is substantially free of hillocks and other defects, whichlead to poorer crystal quality and can be detrimental to deviceperformance. In a specific embodiment, at least 90% of the surface areaof the crystalline material is free from pyramidal hillock structures.

The above sequence of steps provides methods according to an embodimentof the present invention. As shown, the method uses a combination ofsteps including a way of forming a film of crystalline material usingMOCVD. In preferred embodiments, the present invention includes amulti-flow technique provided at atmospheric pressure for formation ofhigh quality gallium nitride containing crystalline films, which havesurface regions substantially smooth and free from hillocks and otherdefects or imperfections. Other alternatives can also be provided wheresteps are added, one or more steps are removed, or one or more steps areprovided in a different sequence without departing from the scope of theclaims herein. Additionally, the various methods can be implementedusing a computer code or codes in software, firmware, hardware, or anycombination of these. In other embodiments, the present MOCVD tool canbe modified, updated, varied, or combined with other hardware,processing, and software.

The above sequence of steps provides a method according to an embodimentof the present invention. In a specific embodiment, the presentinvention provides a method and resulting crystalline material that issubstantially free from hillocks and the like for improved deviceperformance. Other alternatives can also be provided where steps areadded, one or more steps are removed, or one or more steps are providedin a different sequence without departing from the scope of the claimsherein.

FIG. 3 is a simplified diagram illustrating a wurtzite unit cellstructure including various miscuts/offcuts in +/−c/a planes accordingto one or more embodiments of the present invention. This diagram ismerely an example, which should not unduly limit the claims herein. Oneof ordinary skill in the art would recognize other variations,modifications, and alternatives. As shown, the wurtzite unit cellcomprises gallium nitride material, and illustrates relativeorientations of the non-polar m-plane and non-polar a-plane.Additionally, the c-plane is also illustrated for reference purposes. Ina specific embodiment, the curved arrows illustrate tilt directions formiscut or offcut orientations from an m-plane toward the c-plane and/ora-plane.

To prove the operation and method of the present invention, we performedvarious experiments. These experiments are merely examples, which shouldnot unduly limit the scope of the claims herein. As an example, FIG. 4Aand FIG. 4B present optical micrograph images of the resulting surfacemorphology in epitaxial films grown with the use of H₂ carrier gas on(a) an on-axis nonpolar (10-10) GaN substrate and (b) on a nonpolar(10-10) GaN substrate with a substantial miscut toward the a-plane. FIG.5 presents images of the resulting surface morphology in epitaxial filmsgrown with the use of H₂ carrier gas on nonpolar (10-10) GaN substrateswith a varying degree of miscut toward the c-plane. FIG. 6A, FIG. 6B,and FIG. 6C presents images of the resulting surface morphology inepitaxial films grown on a nominally on-axis nonpolar (10-10) GaNsubstrate with the use of N₂ carrier gas. Further details of ourexperiments are shown below.

In this example, we understood that the hillocking can be controlledwith choice of carrier gases (N₂ or H₂) or a mixture thereof and/or withthe choice of slightly off-axis (e.g., miscut or offcut or formation(e.g., grinding, polishing etching, or other shaping processes) nonpolar(10-10) crystal planes. In particular, the hillocking begins todisappear when the substrate is miscut slightly toward the positive ornegative a-plane. See, for example, FIG. 4A and FIG. 4B. For miscutsgreater in magnitude than +/−0.3 degrees toward the a-plane, theepitaxial layers became smooth using a carrier gas of H₂, which has beenknown. Unexpectedly, we have discovered a marked double peak in the400-440 nm emission spectra (as measured by photoluminescence andelectroluminescence) when the a-plane miscut reaches the required amountto achieve smooth morphology. This could be a useful phenomenon in otherdevices, but is not desirable for laser fabrication in the 400-440 nmwavelength range. It is possible that larger miscuts would eliminate thedouble peaked spectra and would offer some other benefit. For example,the double peak is not observed when the composition of the lightemitting layer(s) is adjusted for emission in the 480 nm range. Theseand other limitations have been overcome using the methods and resultingstructures claimed and described herein.

In an effort to achieve smooth epitaxial layers with no double peak withan emission spectra around 405 nm, positive and negative miscuts towardthe c-plane (0001) were explored using growth techniques with H₂ as thecarrier gas. See, for example, FIG. 5. It was found that when smallpositive miscut angles toward the plane c-plane (0001) were used,hillocking was not suppressed and may actually have become more severe.However, when using negative miscut angles toward the c-plane (0001),the hillocking began to disappear when the miscut angle was greater inmagnitude than about negative 0.3-0.5 degrees, and would besubstantially free from hillocks when the angle was greater in magnitudethan about negative 0.6 degrees. In some examples, it was discovered tohigh quality epitaxial growth could not be achieved using small positiveoffcuts. There was no double peak observed in the photoluminescence orelectroluminescence spectra demonstrating a promising approach toachieving smooth epitaxial layers along with high quality light emittinglayers for use in optical devices such as laser diodes and lightemitting diodes.

In addition to substrate miscut, choice of MOCVD carrier gas was alsoexplored. It was found that when all H₂ is replaced by N₂ in the carriergas, smooth relatively defect free epitaxy could be achieved on nonpolarsubstrates with nominally non-miscut (0.0+/−0.1 deg) toward the a-planeand c-plane. See, for example, FIG. 6A, FIG. 6B, and FIG. 6C. It isrelatively uncommon to use all N₂ as the carrier gas when growing p-typeGaN due to reduced dopant incorporation in the lattice, which increasesresistance of the material and degrades device properties. To date, wehave fabricated laser devices demonstrating high performance using allN₂ as the carrier gas in the n-cladding, active region, p-cladding, andeven p-contact layer. The device did not demonstrate forward voltageshigher than those grown using all H₂ as the carrier gases. Additionally,we believe that by using the appropriate mixture of H₂ and N₂ in thecarrier gas along with the appropriate negative miscut toward thec-plane, smooth epitaxial layers can be achieved that will exhibitp-type GaN electrical properties equal to those grown in all H₂. Furtherexamples of film growth and quality in reference to selected miscuts oroffcuts are described throughout the present specification and moreparticularly below.

FIG. 7A and FIG. 7B show an effect of large negative miscut of about 5degrees toward (0001) according to an example of the present invention.The optical micrographs show two examples of smooth, hillock-freesurface morphology using H₂ carrier gas for growth of n-type claddinglayers on GaN substrates with a negative about 5 degree offcut fromm-plane toward c-plane. In this example, the surface is characterized bythe semipolar orientation (60-6-1).

FIG. 8A and FIG. 8B show an effect of large negative miscut of about 7degrees toward (0001) according to an example of the present invention.The optical micrographs show two examples of smooth, hillock-freesurface morphology using H₂ carrier gas for growth of n-type claddinglayers on substrates with a negative about 7 degree offcut from m-planetoward c-plane. In this example, the surface is characterized by thesemipolar orientation (40-4-1).

FIG. 9A and FIG. 9B show an effect of large negative miscut of about 10degrees toward (0001) according to an example of the present invention.The optical micrographs show two examples of smooth, hillock-freesurface morphology for (a) growth of n-type cladding layer using H₂carrier gas and (b) growth of n-type cladding layer using N₂ carrier gason substrates with a negative ˜10 degree offcut from m-plane towardc-plane. In this example, this surface is characterized by the semipolarorientation (30-3-1).

FIG. 10A and FIG. 10B show an effect of large negative miscut of about15 degrees toward (0001) according to an example of the presentinvention. The optical micrographs show two examples of smooth,hillock-free surface morphology using H₂ carrier gas for growth of then-type cladding layers on substrates with a negative 15 degree offcutfrom m-plane toward c-plane. In this example, the surface ischaracterized by the semipolar orientation (20-2-1).

FIG. 11A and FIG. 11B show an effect of large positive miscut of about10 degrees toward (0001) according to an example of the presentinvention. The optical micrographs show two examples of smooth,hillock-free surface morphology using H₂ carrier gas for growth ofn-type cladding layers on substrates with a positive about 10 degreeoffcut from m-plane toward c-plane. In this example, the surface ischaracterized by the semipolar orientation (30-31).

FIG. 12A and FIG. 12B show an effect of large positive miscut of about15 degrees toward (0001) according to an example of the presentinvention. The optical micrographs show four examples of smooth,hillock-free surface morphology using H₂ carrier gas for growth ofn-type cladding layers on substrates with a positive about 15 degreeoffcut from m-plane toward c-plane. In this example, the surface ischaracterized by the semipolar orientation (20-21).

FIG. 13 shows an effect of large positive miscut of about 20 degreestoward (0001) according to an example of the present invention. Opticalmicrographs showing example of smooth, hillock-free surface morphologyusing H₂ carrier gas for growth of n-type cladding layers on substratewith a positive about 20 degree offcut from m-plane toward c-plane. Inthis example, the surface is characterized by the semipolar orientation(30-32).

FIG. 14A and FIG. 14B show atomic force microscopy (AFM) surfacemorphology of growth for H₂ and N₂ carrier gas on (20-21) according toan example of the present invention. As shown are 25 μm×25 μm atomicforce microscope images of n-type cladding growth on (20-21) using an(a) all H₂ type carrier gas showing an RMS roughness of 0.49 nm andusing (b) all N₂ type carrier gas with an RMS roughness of 0.10 nm. Thesubstantially reduced roughness associated with N₂ carrier gas may bedesirable for optical device performance.

FIG. 15A and FIG. 15B show AFM surface morphology for growth with N₂carrier gas on (20-21) according to an example of the present invention.As shown, the visuals include 5 μm×5 μm atomic force microscope imagesof n-type cladding growth on (20-21) using an all N₂ type carrier gaswith an RMS roughness of 0.10 nm. The substantially reduced roughnessassociated with N₂ carrier gas may be desirable for optical deviceperformance.

FIG. 16A and FIG. 16B shows AFM surface morphology for multilayer growthusing a first N₂ carrier gas and a second H₂ carrier gas on (20-21)according to an example of the present invention. As shown, thephotographs illustrate 5 μm×5 μm atomic force microscope images ofn-type cladding growth on (20-21) comprising two layers. The first layeris approximately 1 μm thick using N₂ carrier gas and the second layer isapproximately 1 μm thick using an H₂ carrier gas. Since the RMSroughness is maintained at 0.1 nm, this demonstrates that smooth H₂growth can be achieved by initiating with N₂ growth.

In a specific embodiment, the present invention provides a high qualityfilm overlying an off-cut surface having a semipolar crystalorientation. In an example, the film is grown at pressure ranging from650-850 Torr. In an example, the present invention provides forprocessing such that a substantial portion of the n-cladding region isepitaxially grown using primarily an N₂ type carrier gas, and possiblywith other species. The present invention can also provide for a growthpressure of at least an active layer is above atmospheric pressureranging from 800 Torr to 1600 Torr. In a specific example, the presentinvention also includes an offcut between negative 13 degrees andnegative 17 degrees from m-plane toward a c-plane, an offcut between 2degrees and 8 degrees from m-plane toward an a-plane, atmospheric growthpressure of between 650 Torr to 850 Torr, a device capable of emissionin the blue region of 420 nm to 485 nm or in the green region of 500 nmto 550 nm. The high quality film is provided for a laser device alignedin the projection of the c-direction, but can be others.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A method of forming an optical device, saidmethod comprising: growing by MOCVD at least one gallium- andnitrogen-containing epitaxial layer over a crystalline surface plane ofa gallium- and nitrogen-containing substrate, said at least oneepitaxial layer comprising: at least one light emitting layer configuredto emit luminescence substantially free from a double-peak emissionspectrum; and a surface region being substantially free from hillocks,wherein said crystalline surface plane is substantially oriented alongthe (30-3-1) direction.
 2. The method of claim 1, wherein growing saidat least one epitaxial layer comprises growing said light-emitting layerat a temperature ranging from about 700 Degrees Celsius to about 1200Degrees Celsius.
 3. The method of claim 1, wherein growing said at leastone epitaxial layer comprises growing said light-emitting layer in acarrier gas selected from a group comprising H₂, N₂, and a mixture of H₂and N₂.
 4. The method of claim 1, wherein said growing said at least oneepitaxial layer comprises growing said at least one epitaxial layer atan atmospheric pressure.
 5. The method of claim 4, wherein saidatmospheric pressure is within a range from about 700 Torr to about 800Torr.
 6. The method of claim 1, wherein said at least one light-emittinglayer comprises at least one quantum well having a thickness of at least3 nm.
 7. The method of claim 1, wherein said at least one light-emittinglayer comprises at least one quantum well having a thickness of at least5 nm.
 8. The method of claim 1, wherein at least 90% of said surfaceregion is free from hillocks.
 9. The method of claim 1, wherein saidoptical device is one of a laser and a light emitting diode.
 10. Themethod of claim 1, wherein growing said at least one epitaxial layercomprises growing multiple gallium- and nitrogen-containing epitaxiallayers comprising n-type regions, p-type regions, and at least onequantum well.